The subject matter of the present invention pertains to means for minimizing fluctations in the angular velocity of a syncronous motor shaft, and in particular the shaft of a step or stepping motor. Simply stated, a stepping motor is a synchronous motor whose output shaft rotates in incremental response to a series of changes in an input drive signal. When properly controlled, the output increments or steps are always equal in number to the number of input signal changes. For a basic understanding of the theory and operation of such motors, see, for example, Benjamin C. Kuo, "Theory and Applications of Step Motors," West Publishing Co., St. Paul, 1974, all pertinent parts of which are incorporated herein by this reference.
As is known to the art, stepping motors have been employed for some time in a wide range of control applications. More recently, they have found use in practially all types of computer peripheral equipment, such as printers, tape drives, memory access mehanisms, and incremental plotters. Being inherently discrete-motion devices, stepping motors are compatible with digital control techniques and any positional error introduced during their operation is noncumulative. Moreover, it is possible to achieve accurate position and speed control in an open-loop environment. When operating in such an environment, a stepping motor may experience three major modes of operation; discrete incremental motion (stepping), continuous unidirectional motion (slewing), and, between stepping and slewing, transitional. In the stepping mode, the rotor element of the motor comes to rest between each incremental movement, in the slewing mode, it does not, and the motor behaves very similar to a synchronous motor. In the transitional mode, shaft motion is somewhat erratic and unpredictable.
A common problem with stepping motors operating in the slewing mode is the tendency of their rotating shafts to turn with a fluctuating angular velocity, a phenomenon similar to the hunting characteristics of a synchronous motor. Such fluctuations are oscillatory in nature and tend to occur whenever the frequency of the motor drive or excitation current is equal to or a harmonic of a natural or resonant frequency of the spring/mass equivalent of the motor and its associated load. The amplitude of the velocity fluctuations is a function of both the amplitude and the frequency of the drive current supplied to the motor.
If not corrected or reduced to an insignificant level, the fluctuations in angular shaft velocity will introduce intolerable nonlinarities into the operation of the particular piece of equipment being controlled by the motor. Such correction or reduction is especially important in the field of incremental plotters where such nonlinearities severely limit the ability of the device to produce high-resolution graphics.
Known methods for controlling oscillations in a stepping motor system are directed generally to the damping of oscillations during the incremental or stepping mode of operation as opposed to the continuous motion or slewing mode. A number of such methods are outlined in the Kuo reference cited above and include the use of mechanical inertia dampers, the use of electronics switching schemes markedly dissimilar from that of the present invention, and the modification of physical and electrical motor parameters. Other means and methods for controlling the operation of stepping motors are disclosed in Cannon U.S. Pat. Nos. 4,126,821, Schaff 4,104,574, Pritchard 4,087,732, and Leenouts 3,908,195.